linux-kernel - Re: [PATCH] riscv: return -ENOSYS for syscall -1

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Date:   Wed, 23 Dec 2020 08:24:04 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Tycho Andersen <tycho@...ho.pizza>
Cc:     Andreas Schwab <schwab@...e.de>,
        David Abdurachmanov <david.abdurachmanov@...ive.com>,
        linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-kernel@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>
Subject: Re: [PATCH] riscv: return -ENOSYS for syscall -1

On Tue, Dec 22, 2020 at 09:22:19AM -0700, Tycho Andersen wrote:
> On Mon, Dec 21, 2020 at 11:52:00PM +0100, Andreas Schwab wrote:
> > Properly return -ENOSYS for syscall -1 instead of leaving the return value
> > uninitialized.  This fixes the strace teststuite.
> > 
> > Fixes: 5340627e3fe0 ("riscv: add support for SECCOMP and SECCOMP_FILTER")
> > Signed-off-by: Andreas Schwab <schwab@...e.de>
> > ---
> >  arch/riscv/kernel/entry.S | 9 +--------
> >  1 file changed, 1 insertion(+), 8 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> > index 524d918f3601..d07763001eb0 100644
> > --- a/arch/riscv/kernel/entry.S
> > +++ b/arch/riscv/kernel/entry.S
> > @@ -186,14 +186,7 @@ check_syscall_nr:
> >  	 * Syscall number held in a7.
> >  	 * If syscall number is above allowed value, redirect to ni_syscall.
> >  	 */
> > -	bge a7, t0, 1f
> > -	/*
> > -	 * Check if syscall is rejected by tracer, i.e., a7 == -1.
> > -	 * If yes, we pretend it was executed.
> > -	 */
> > -	li t1, -1
> > -	beq a7, t1, ret_from_syscall_rejected
> > -	blt a7, t1, 1f
> > +	bgeu a7, t0, 1f
> 
> IIUC, this is all dead code anyway for the path where seccomp actually
> rejects the syscall, since it should do the rejection directly in
> handle_syscall_trace_enter(), which is called above this hunk. So it
> seems good to me.

That change really needs to be documented in the commit log, or even
better split into a separate patch (still documented of course!).